This invention relates to a method and apparatus for driving a fastener into a workpiece such as drywall, ceiling or the like for framing a building. More particularly, this invention relates to driving a fastener into a workpiece with a power tool at a high velocity without damaging the workpiece.
Pneumatic fastener driving tools have been widely used in the building industry to join workpieces to other structural components. These tools typically employ a piston-type drive mechanism, which during its driving stroke, engages one of a series of fasteners fed to the tool to force the fastener into the workpiece to be joined.
Many devices have been constructed using these tools, in particular pneumatic-type, to secure drywall to framing studs in constructing walls or ceilings. This drywall and framing stud combination is generally referred to as a workpiece. To construct these walls, four main criteria must be met so that the wall will comply with local building codes. First, the head of the fastener must be recessed in the drywall to a predetermined amount. Second, each drywall sheet must be held firmly to the framing studs. Third, it is preferable that the drywall be slightly indented around the recessed fastener so that the fastener's head may be later concealed with various known sealing compounds deposited in and around each of the indentations. Fourth, the nail must be inserted into the drywall without any damage, such as ripping or tearing, to the paper on the surface of the drywall.
Several pneumatic tools have been proposed to perform these some of these functions in a single operation. Examples of tools for performing some of the aforementioned techniques are disclosed in U.S. Pat. Nos. 2,918,675, 3,774,293, 4,566,619, 4,778,094 and 3,040,327. However, many of the prior art tools have not satisfactorily accomplished these multiple functions. The main reasons for the failure of these pneumatic tools to satisfactorily drive and recess a fastener, while indenting the drywall without ripping or tearing the paper surface of the drywall, is due to the fastener being forced into the drywall at an angle less than 90.degree. relative to the surface of the drywall. Other causes of ripping or tearing are due to the head of the fastener penetrating the surface of the drywall by too great of an amount.
Tools have been designed in a manner which attempts to limit the penetration of the fastener's head into the drywall with a resilient member that slows down the force of the piston hitting the fastener. See U.S. Pat. Nos. 2,918,675 to Smith, 3,774,293 to Golsch, 4,566,619 to Kleinholz and 4,778,094 to Fishback. The forward movement of the piston in such designs is then completely stopped with metal-to-metal contact between the piston and a portion of a dimpler mechanism. This metal-to-metal contact between the piston and a portion of the dimpling part reduces the durability of the tool and also results in stresses in the tool's metal components, thereby lowering the tool's life expectancy.
There are other types of driving tool designs that use a resilient material to stop the forward velocity of the piston. These tools are susceptible to the fastener penetrating the workpiece by more than a predetermined distance resulting in ripping or tearing of the paper on the surface of the drywall. Specifically, the studs are often made with either very hard wood or very soft wood. When the stud is made with hard wood, the fastener encounters more resistance when penetrating the stud than when the fastener is forced to penetrate soft wood. Consequently, with a hard wood stud a resilient material reduces the velocity of the piston too quickly, resulting in the fastener not being driven all the way into the workpiece. On the other hand, with a soft wood stud, the fastener is driven into the drywall too great of a distance, resulting in a ripping or tearing of the paper on the surface of the drywall.
In addition to the fastener's penetration depth not remaining consistent when forced into drywall, the angle at which the fastener penetrates the workpiece may vary with some designs. If the fastener does not penetrate the workpiece at an angle perpendicular to the surface of the workpiece, a ripping or tearing of the paper surface may also result.
Other prior art devices have been designed in an attempt to drive tacks into fabric or the like at nearly perpendicular angles. Examples of these devices are disclosed in U.S. Pat. Nos. 3,854,648, 4,195,762 and 4,252,260. These patents disclose tack-dispensing nozzle assemblies that have wing portions that are pivotally mounted to a nozzle holder. A tack is projected through a channel in the wing portion and out a forward end of the nozzle. When the tack is driven, a circular tackhead forces apart the wing portion. The circular tackhead is centered on the wing portion to maintain alignment while the tack is being driven into the upholstery. However, the heads of fasteners used with drywall are typically very small. Further, the head may have a non-circular shape resulting in the nailhead not being properly centered with the wing portion and the nail being driven into the wood at skewed angles. When nails are driven into drywall at skewed angles, the paper on the surface of the drywall may rip or tear, requiring removal and replacement of the drywall.